This invention relates generally to magnetic resonance (MR) imaging techniques. In particular, the invention relates to seamlessly switching between fast MR fluoro and MR diagnostic imaging modes.
Magnetic Resonance Imaging (MRI) is a widely accepted and commercially available technique for obtaining digitized visual images representing the internal structure of objects (such as the human body) having substantial populations of atomic nuclei that are susceptible to nuclear magnetic resonance (MR) phenomena. In MRI, nuclei in the body of a patient to be imaged are polarized by imposing a strong main magnetic field (B0) on the nuclei. The nuclei are excited by a radio frequency (RF) signal at characteristic MR (Larmor) frequencies. By spatially distributing localized magnetic fields surrounding the body and analyzing the resulting RF responses from the nuclei, a map or image of these nuclei responses as a function of their spatial location is generated and displayed. An image of the nuclei responses provides a non-invasive view of a patient""s internal organs and of other tissues.
As shown in FIG. 1, an MR imaging system typically includes a magnet 10 to impose the static magnetic field (B0), gradient coils 12 for imposing spatially distributed gradient magnetic fields (Gx, Gy, and Gz) along three orthogonal coordinates, and RF coils 14 and 16 to transmit and receive RF signals to and from the selected nuclei of the body being imaged. The patient 18 lies on a patient table 20 such that a portion of the patient to be imaged is moved, in three-dimensions, into an xe2x80x9cimaging volumexe2x80x9d between the magnet and coils, which defines a field of view (FOV) of the MRI system.
The MRI system operator controls the system through a computer workstation 22 with a keyboard, screen and other operator input/output devices. The MRI system operator positions the patient within the imaging volume using a movable table 20, and selects one or more imaging parameters, such as: (a) imaging technique, e.g., diagnostic MRI, fast-MRI, MR fluoroscopy, and MR vascular imaging; (b) pulse sequence, e.g., spin echo, field echo, inversion recovery, fast spin echo and fast field echo; (c) imaging modes, e.g., multi-slice MR scans, multi-slab three-dimensional (3D) scans, multi-echo scans, multi-coverage (to cover an area greater than that covered by a single scan), and multi-angle acquisition (multiple groups of slices with different angles in the same TR); (d) fat suppression and separation techniques, and (e) artifact suppression techniques.
After the desired imaging parameters have been selected, the MRI system is programmed to scan the patient with one or more respectively corresponding pulse sequence(s) of RF pulses, slice-selection and phase encoding magnetic gradient pulses and read-out magnetic gradient field pulses. When the diagnostic scan is initiated, a predetermined pulse-sequence is repeated to generate a series of NMR RF responsive signals from the excited nuclei of the patient. The MRI system analyzes these signals and generates images of the internal organs and tissues of the patient based on the responsive RF signals.
The diagnostic MR image so generated is influenced by the selected imaging mode and imaging parameters. When the MR image is unsatisfactory or when a doctor wants to see an image from another viewpoint, another MR image is generated by adjusting the desired mode and/or selected image parameter values and then repeating the whole imaging procedure. For instance, if the contrast between two or more objects of interest shown in an MR image is not optimal, the imaging parameters for MR imaging must be adjusted to obtain proper contrast. Similarly, if the doctor judges that an axial picture obtained by MR imaging a certain portion of the head did not provide good diagnostic information, another MR image from another view point must be selected and generated.
The operator selects the desired imaging parameters before an MR image is generated. The selection of the imaging parameters determines image location, slice orientation, image quality, viewpoint and other features. It is difficult to optimally select the many imaging parameters before any image is generated. The resulting images generated from the initial parameter selections are sometimes inadequate because the selected imaging parameters are, in hindsight, less than optimal. Only by viewing an actual image does it become evident that some or all of the imaging parameter selections should be reset. However, the process of generating an MR image, resetting the imaging parameters and generating another image is excessively time consuming (e.g., several minutes), especially with diagnostic mode MR imaging techniques that require long scanning periods.
Accordingly, there is a long-felt need for a system in which the operator may quickly preview initial MR images based on the parameter selections that have currently been entered in the workstation 22. There is also a need for a technique for adjusting the diagnostic imaging parameters in view of the initial quick view images and before actually performing a longer diagnostic MRI mode. There is no known process for rapidly initially confirming that imaging parameters are properly selected for normal diagnostic MR imaging, before going through a complete diagnostic mode imaging scan.
While some real-time MR imaging techniques have been developed, they have not been applied as a tool for selecting normal (and hence slower) diagnostic scan imaging parameters. Examples of a real-time imaging process using a fast fluoro-mode imaging procedure are disclosed in U.S. Pat. Nos. 5,184,074 (Kaufman et al.) 5,898,305 (Kokubun), and 4,830,012 (Riederer).
In fluoro-mode imaging, near real-time MR images are generated using a short repetition time MRI pulse sequence. Fluoro-mode imaging has conventionally been used for MR imaging where fast imaging times are needed, such as for patient positioning within the MR imaging volume and for interventional MR imaging. While it provides fast images, fluoro-mode imaging is viewed as being inadequate for most diagnostic MR imaging purposes. See U.S. Pat. No. 5, 713,358, col. 2, Ins. 1-33. Moreover, fluoro-mode imaging has not been used as a tool to expedite the selection or confirmation of imaging parameters for slower diagnostic imaging.
The present invention satisfies the above-described need for quickly confirming and adjusting diagnostic MR imaging parameters, such as, image position, orientation and alignment, and field of view (FOV), before taking the time to generate a normal diagnostic MR image. The invention enables an MRI system operator and/or associated medical practitioner to obtain quick initial MR images to confirm that the optimal imaging parameters are properly selected. In addition, the operator is able to quickly adjust the imaging parameter settings during the fast-imaging mode, if the initial settings are found to be improperly set upon viewing a fast image. Having used the fast imaging mode to confirm and/or further adjust MR imaging parameter settings, the MRI system operator can commence normal diagnostic imaging with confidence that proper image settings have been entered in the system. Accordingly, the invention provides a way to quickly confirm and adjust imaging settings for subsequent use in normal diagnostic imaging.
Using an embodiment of the invention, an MRI system operator: (a) positions a patient within the MRI imaging volume and uses a rough targeting system (e.g., projected optical cross-hairs, fluoro MR imaging, rapid batch location and other conventional acquisition locators) to align the patient within that volume; (b) selects initial imaging parameters for a particular diagnostic imaging mode; (c) switches the MRI system to a respectively corresponding xe2x80x9cfluoro-modexe2x80x9d to generate a fast MR image that is used to confirm and adjust the alignment of a designed organ or other imaging target within the MRI field of view and to adjust pulse sequence and other parameters; and (e) switches back to a normal diagnostic imaging mode, using the imaging parameter selections confirmed or adjusted during fluoro-mode, to generate a complete MR image.
When switching from diagnostic-mode to fluoro-mode, the MRI system saves the original diagnostic image parameter settings that were selected by the operator for diagnostic imaging. These same imaging parameter settings that were earlier selected for diagnostic mode imaging are now automatically used (except for those settings which must be adjusted to accommodate fluoro-MRI and its speed) in a thus respectively corresponding fluoro-mode imaging mode. Using the image parameter settings selected for diagnostic imaging in a fast imaging mode provides a quick image that is useful to check the diagnostic image settings.
Not all of a given set of diagnostic image parameter settings can be used for fast imaging. To achieve fast image generation, certain of the image parameter settings are automatically converted for fast fluoro-mode, such as reducing the number of selected sequence steps and sequence slices. While in fluoro-mode, the operator may adjust some image parameter settings that were earlier selected for diagnostic imaging. When the operator then switches from fluoro-mode back to diagnostic mode imaging, the MRI system automatically applies the same imaging parameter settings that were earlier selected for diagnostic mode and that were not adjusted during fluoro-mode (many of which diagnostic settings were used in fluoro-mode imaging, while other diagnostic mode settings were automatically adjusted to correspond to fuoro-mode imaging settings), and applies to the diagnostic settings the adjustments made during fluoro-mode.
In this regard, the MRI system converts the parameter settings that were automatically changed for fluoro-mode, (e.g., number of sequence steps and slices), back to those settings originally selected for diagnostic imaging. In addition, the system also applies to diagnostic imaging parameters the adjustments that were made by the operator during fluoro-mode. Accordingly, the operator does not have to manually reset the MRI system when switching back-and-forth between diagnostic and fluoro imaging modes. The transition between modes is done effortlessly to expedite optimization of the diagnostic imaging mode settings.